Above are three drawings by contemporary Swiss artist Andre Beuchat. Here's a link to a gallery representing his work.

And here, above, is a rendition of Kefka's Tower for one of the Final Fantasy franchises by Yoshitaka Amano, award-winning Japanese illustrator and designer.

Above are three paintings by science fiction's master surrealist, Richard Powers. (I found two vintage Ballantine titles this weekend with Powers illustrations on the covers; an entire post impends.)

And above we have three renderings by the late architect Lebbeus Woods. Woods was interested in architecture, not fantasy, but to date, only one of this projects has ever seen the full light of steel and glass.

Not exactly a tower, but this version of "Isle of the Dead" by Swiss painter Arnold Bocklin seems to fit.